The present invention relates to a process for the preparation of acyl bicyclo[2.2.1]heptenes and/or derivatives thereof. Acyl bicyclo[2.2.1]heptenes are valuable chemicals per se (e.g. as solvents for dyes and as stabilizers for solutions of dyes in hydrocarbons) and are also useful as intermediates, e.g. for compounds used in modern perfumery.
It is known from Journal f. prakt. Chemie, Band 317, Heft 3, 1975, pages 510-514 that 2-acyl bicyclo[2.2.1]hept-5-enes can be obtained by reacting cyclopentadiene with methylvinylketone in the presence of (a halogen substituted) acetic acid or aluminum trichloride. No yields are reported therein. Virtually no reaction was observed, however, when cyclopentadiene was brought together with mesityl oxide in the presence of trichloroacetic acid, the fastest Bronsted acid catalyst mentioned in the reference cited hereinbefore. Apparently, the replacement of the hydrogen atoms at the terminal double bond carbon atom almost completely deactivates the reactivity towards Diels-Alder type reactions.
It is also known from Synthesis, Communications, 1979, pages 270-271, that the Diels-Alder adduct of 1,3-cyclohexadiene and acroleine can be obtained in high yield by stirring the reactants during no less than 40 hours at room temperature in the presence of a Nafion-H (tradename) catalyst (resin based). It should be noted, however, that Nafion-H resins, apart from being expensive, are very sensitive to water and can neither be applied successfully in systems wherein water is present as solvent or as reactant nor in systems wherein water is formed during the reaction.
It is thus an object of the present invention to provide a process for the preparation of Diels-Alder type adducts starting from less reactive dienophiles, which process can be carried out using an aqueous catalytic system.